Diabetes is a group of the diseases characterized by an abnormal glucose homeostasis causing an increased blood glucose and may be divided mainly into two types, i.e., type 1 diabetes mellitus (insulin-dependent diabetes mellitus) and type 2 diabetes mellitus (noninsulin-dependent diabetes mellitus). Among the diabetes, the type 2 diabetes mellitus accounts for the great majority of the diabetes.
Glucosidase as a carbohydrate degrading enzyme (particularly α-glucosidase) is involved in several significant biological processes (including, for example, digestion, biosynthesis of glycoproteins, and lysosomal catabolic reaction of conjugated carbohydrates). Therefore, by using a glucosidase inhibitor such a substance inhibiting the carbohydrate degrading action of such an α-glucosidase, the digestion and absorption of carbohydrates in the intestines and so on can be suppressed. This is expected to be useful for a glucosidase inhibitor as an agent for treating or preventing diabetes.
In the latter half of the 1990s, salacinol having the chemical formula as will be described below has been discovered as a pharmacologically essential substance from a medicinal plant (Salacia reticulate) which has been used in a traditional medicine (ayur veda) of India. Salacinol is reported as having an α-glucosidase inhibitory activity (Patent Document No. 1, Non-Patent Literature Documents 1 and 2).

In addition to salacinol as described above, kotalanol, ponkoranol and so on were discovered as salacinol analogs. These analogs are reported, too, as having a α-glucosidase inhibitory activity (Patent Document 2, Non-Patent Literature Documents 3 and 4). These naturally occurring substances including salacinol have the characteristics that an erythritol-like carbon side chain is connected to the endocyclic sulfur atom at the thiosugar moiety portion forming a sulfonium ion and further the sulfonium ion forms an intramolecular bond with a sulfate anion on the carbon side chain, thereby forming a specific structure with a spiro framework.
Moreover, from a viewpoint of readiness of production or improvements in pharmacological activities, there have been reported cyclic onium compounds which are desulfate esters having no sulfate anion on the carbon side chain of salacinol, including a desulfate ester of salacinol (hereinafter called neosalcinol), as will be described by the following chemical formula (Patent Document 3). These desulfate esters have α-glucosidase inhibitory activities equal to or higher than the naturally occurring substances and are superior in terms of readiness in production such as stability and so on.

Further, recently, 3′-O-methylneoponkoranol (n=0) was reported (Non-Patent Literature Document 5). In this document, it was reported that the methyl derivative have little contributed to improvements in the α-glucosidase inhibitory activity compared with the naturally occurring salacinol. This implies that the introduction of an alkyl group or the like into the hydroxyl group at the 3′-position cannot improve the α-glucosidase inhibitory activity.
[Patent Document 1] Japanese Patent Publication No. 2002-179673.
[Patent Document 2] Japanese Patent Publication No. 2004-323420.
[Patent Document 3] Japanese Patent Publication No. 2005-002051.
[Non-Patent Literature Document 1] Yoshikawa, M., Muraoka, O., et al., Tetrahedron Lett., 1997, 38, 8367.
[Non-Patent Literature Document 2] Yoshikawa, M., Muraoka, O., et al., Bioorg. Med. Chem., 2002, 10, 1547.
[Non-Patent Literature Document 3] Yoshikawa, M., et al., Chem. Pharm. Bull., 1998, 46, 1339.
[Non-Patent Literature Document 4] Yoshikawa, M., Muraoka, O., Heterocycles, 2008, 75, 1397.
[Non-Patent Literature Document 5] Eskandari, R., et al., Bioorg. Med. Chem. Lett., 2010, 20, 5686.